Apparatus for Pervaporation Control in Liquid Degassing Systems

ABSTRACT

A liquid degassing apparatus is arranged to limit pervaporation through a membrane by attenuating pressure oscillations developed by a vacuum pump. The attenuation is obtained through a combination of one or more flow restrictors and added volume chambers fluidly interposed between the degassing chamber and the vacuum pump. The pressure oscillation attenuation may further inhibit cross-contamination of pervaporated solvents among a plurality of distinct degassing chambers.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation in part of U.S. patentapplication Ser. No. 13/072,422, filed Mar. 25, 2011 and entitled“APPARATUS FOR PERVAPORATION CONTROL IN LIQUID DEGASSING SYSTEMS”, thecontent of such application being incorporated herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to liquid degassing systems generally, andmore particularly to a liquid degassing apparatus that is specificallyarranged to minimize pervaporation in a degassing chamber. The apparatusof the present invention may be particularly adapted for control ofpervaporation in degassing systems utilized in liquid chromatographyapplications.

BACKGROUND OF THE INVENTION

Solvent pervaporation through a membrane is a well known phenomenon thathas been harnessed in membrane separation applications. For example, theprior art is rich with examples of the use of solvent pervaporationthrough a membrane for the purpose of concentrating relatively low vaporpressure components on a retentate side of the membrane. In addition,distillation operations utilizing pervaporation through a membrane havebeen performed to selectively recover solvent components on the permeateside of the membrane.

While the beneficial aspects of pervaporation have long been known andutilized in purposeful solvent separation processes, such pervaporationcharacteristics can have significant negative effects in mixed-solventapplications wherein the relative concentrations of the respectivesolvents is desired to be known and/or constant. A particular example ofsuch a mixed-solvent application is in liquid chromatography systems,wherein mobile phases of more than one solvent are used. It has beenrecognized by the Applicants, however, that changes to the relativeconcentrations of the mobile phases can occur over time, therebynegatively affecting the accuracy of chromatographic analysis.

Pervaporation effects are particularly damaging to analytical accuracyin chromatographic systems utilizing relatively low through-put mobilephase volumes, or in instances wherein the chromatographicinstrumentation is only periodically operated without complete flushingof supply lines between each operation. For example, systems thatutilize mobile phase flow rates of on the order of nanoliters ormicroliters per hour are at risk of having the relative concentrationsof the solvents making up the mobile phase being substantially modifiedduring analyte transportation through the chromatographicinstrumentation.

In particular, liquid chromatography systems typically employ degassingchambers in which the liquid mobile phase is exposed to a degassingenvironment through a gas-permeable, liquid-impermeable membrane. Such adegassing environment may be, for example, relatively low absolutepressure maintained by evacuation pumps, or relatively low targetmaterial partial pressures in a sweep fluid passed through a permeateside of a degassing chamber. Typically, degassing operations have beenarranged and controlled to maximize degassing performance on the mobilephase passing through the degassing chamber. To do so, vacuum pumps aretypically programmed to maintain relatively low absolute pressures onthe permeate side of the membrane, or, in the cases of a sweep fluid, asweep fluid containing little or no concentration of the targeted gasspecies being withdrawn from the mobile phase. In both cases, a targetgas concentration gradient is maintained to drive target gas transferthrough the membrane to the permeate side. A result of maintaining sucha large target gas concentration gradient at all times in the degassingchamber can be pervaporation. Specifically, relatively long residencetime of mobile phase within the degassing chamber having a permeate sidemaintained at the conditions described above has a tendency to cause achange in relative solvent concentrations as a result of pervaporationthrough the membrane of relatively higher vapor pressure solventcomponents. As a consequence, the mobile phase on the retentate side ofthe degassing chamber can become concentrated in relatively lower vaporpressure component materials, particularly if such mobile phase has arelatively high residence time within the degassing chamber, or if thepermeate side of the degassing chamber is conducive to ongoingpervaporative effects.

It is therefore an object of the present invention to provide anapparatus for controlling pervaporation of a mobile phase having two ormore component materials through a membrane.

It is another object of the present invention to provide an apparatusfor establishing an environment on the permeate side of a membrane thatis effective in limiting pervaporation through the membrane of a mobilephase having two or more component materials.

It is a further object of the present invention to provide an apparatusfor attenuating pressure oscillations in a vacuum degassing system.

It is a still further object of the present invention to inhibitcross-contamination of pervaporated solvent among a plurality ofdistinct degassing chambers in a vacuum degassing system.

SUMMARY OF THE INVENTION

A liquid degassing apparatus is arranged to limit pervaporation througha membrane, in one aspect, by establishing a pervaporation control spaceat a permeate side of the membrane. The pervaporation control space isdefined in a vacuum chamber between the membrane and a shield member.The shield member may be substantially gas and liquid impermeable tomaintain an environment in contact with the permeate side of themembrane that is relatively rich in solvent vapor concentration, therebylimiting further liquid pervaporation across the membrane.

A liquid degassing apparatus may also be arranged to limit pervaporationby attenuating pressure oscillations generated by a vacuum pump. Theattenuation may be accomplished through a pneumatic filtration in theevacuation line between the degassing chamber and the vacuum pump.Pneumatic filtration may be carried out through one or more of flowrestriction and added volume, and may be particularly effective whenfluidly interposed between a pressure sensor and the controlled vacuumpump.

In one embodiment of the invention, the liquid degassing apparatusincludes a body defining a chamber having a liquid inlet and a liquidoutlet, and a vacuum port. A gas-permeable, liquid impermeable membraneis disposed in the chamber to separate the chamber into a permeate sideand a retentate side, wherein the retentate side of the chamber is inliquid communication with the liquid inlet and the liquid outlet. Thepermeate side of the chamber is in fluid communication with the vacuumport. The liquid degassing apparatus further includes a shield memberthat is disposed in the permeate side of the chamber, and is interposedbetween the membrane and the vacuum port. The shield member defines asubstantially closed pervaporation control space between the membraneand the shield member. The pervaporation control space assumes a volumethat is not greater than about 30 times the volume of the retentate sideof the chamber. The liquid degassing apparatus also includes a pump forevacuating the permeate side of the chamber through the vacuum port.

In some embodiments, the liquid degassing apparatus further includes aflow restrictor in a fluid path between the chamber and the pump, withthe flow restrictor being suitable for creating a pneumatic pressureoscillation dampener having a time constant that is larger than anoscillation rate of the vacuum pump.

In another embodiment, the liquid degassing apparatus includes adegassing module for degassing a liquid conveyed through a chamberthereof, with the degassing module including gas-permeable,liquid-impermeable membrane separating the chamber into a permeate sideand a retentate side. The retentate side of the chamber is in liquidcommunication with a liquid inlet and a liquid outlet for the liquid,and the permeate side of the chamber is in fluid communication with avacuum port of the degassing module. The apparatus further includes avacuum pump fluidly coupled to the vacuum port of the degassing modulethrough a degassing line to evacuate the permeate side of the chamber. Apressure sensor is included for sensing pressure of the permeate side ofthe chamber, and to transmit signals to a controller communicativelycoupled to the pressure sensor and the vacuum pump for controllingvacuum pump operation responsive to the signals received from thepressure sensor indicating pressure of the chamber. The apparatusfurther includes a pneumatic filtration apparatus fluidly interposedbetween the vacuum pump and the pressure sensor within the degassingline, with the pneumatic filtration apparatus including one or more of afirst flow restrictor and a volume exchange chamber defining a volumeopen to the degassing line. The pneumatic filtration apparatus iscapable of attenuating pressure oscillations developed in the degassingline by the vacuum pump, wherein the attenuation provided by thepneumatic filtration apparatus is at least 10%.

A further liquid degassing apparatus includes a plurality of degassingmodules for degassing one or more liquid compositions, with each of thedegassing modules including a chamber separated by a gas-permeable,liquid-impermeable membrane into a permeate side and a retentate side.The retentate sides of the chambers may be liquidly disconnected fromone another, while a manifold fluidly connects the permeate sides of thechambers through respective individual degassing lines individuallyextending between the manifold and respective vacuum ports in fluidcommunication with the permeate sides of the chambers.

The manifold further fluidly connects the individual degassing lineswith a main degassing line at a connection. The apparatus also includesa vacuum pump fluidly coupled to the main degassing line for evacuatingthe permeate sides of the chambers. A buffer chamber may be fluidlyinterposed between the connection and at least one of the degassingmodules along a respective individual degassing line, wherein the bufferchamber defines a first volume open to the individual degassing line.The first volume is of a magnitude exceeding an intake volume, whereinthe intake volume may be defined as: [(P_(o)/P_(set))×(100%)]×V_(ch).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a liquid degassing apparatus ofthe present invention;

FIG. 2 is an enlarged view of a portion of the schematic illustration ofFIG. 1;

FIG. 3 is an enlarged view of a portion of the schematic illustration ofFIGS. 1 and 2;

FIG. 4 is a schematic view of a portion of the liquid degassingapparatus of the present invention;

FIG. 5 is a schematic view of a portion of the liquid degassingapparatus of the present invention;

FIG. 6 is a schematic view of a portion of the liquid degassingapparatus of the present invention;

FIG. 7 is a schematic view of a portion of the liquid degassingapparatus of the present invention;

FIG. 8 is a schematic view of a liquid degassing apparatus of thepresent invention; and

FIG. 9 is a schematic view of a liquid degassing apparatus of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects and advantages enumerated above together with other objects,features, and advances represented by the present invention will now bepresented in terms of detailed embodiments described with reference tothe attached drawing figures which are intended to be representative ofvarious possible configurations of the invention. Other embodiments andaspects of the invention are recognized as being within the grasp ofthose having ordinary skill in the art.

With reference now to the drawing figures, and first to FIG. 1, apervaporation control system 10 in a liquid degassing apparatus 8 isarranged to provide a minimal volume pervaporation control space 12, aswill be described in greater detail hereinbelow. Liquid degassingapparatus 8 includes a body 14 which defines a chamber 16 through whichpervaporation control system 10 operably extends. Body 14 may compriseone or more component parts, and defines a liquid inlet 18 and a liquidoutlet 20. Body 14 further defines a vacuum port 22 which establishesfluid communication between chamber 16 and a vacuum pump 17 coupled toport 22.

Body 14 may be fabricated from a non-porous, non-absorptive materialsuch as polyphenylene sulfide, PEEK, non-porous metal, or non-porousglass. Such materials inhibit solvent pervaporation through an exteriorwall thereof. In the embodiment illustrated in FIG. 1, body 14 defines avacuum chamber 16 that is separated into a permeate side 30 and aretentate side 32, with the retentate side 32 of chamber 16 being inliquid communication with liquid inlet and outlet 18, 20 at liquid inletconnection 26 and liquid outlet connection 28, respectively. Permeateside 30 of chamber 16 is in fluid communication with vacuum port 22. Inthe embodiment illustrated in FIG. 1, membrane 24 is in the form of atube for conveying liquidous material through chamber 16 from liquidinlet connection 26 to liquid outlet connection 28. As such, retentateside 32 of membrane 24 is the lumen of the tube formed by membrane 24,and permeate side 30 is the space of chamber 16 external to the tubularmembrane 24.

Membrane 24 may be disposed in chamber 16 in a variety ofconfigurations, and being limited only by the requirement that membrane24 effectively contain the liquid portion of a mobile phase enteringchamber 16 at inlet connection 26 on a retentate side of membrane 24.Accordingly, membrane 24 may be arranged in any suitable configurationfor separating chamber 16 into a permeate side 30 and a retentate side32, with the retentate side of chamber 16 being in liquid communicationwith liquid inlet and outlet connections 26, 28, and the permeate side30 of chamber 16 being in fluid communication with vacuum port 22.

Membrane 24 may preferably be gas-permeable, liquid-impermeable so as tosubstantially inhibit liquidous material from passing therethrough.Accordingly, membrane 24 may be fabricated from a variety of materials,including flouropolymers such as PTFE, ePTFE, and perfluorinatedcopolymer available from E.I. du Pont de Nemours and Company under thetrade name Teflon AF®. An example construction of a tubular membrane ina vacuum degassing chamber is described in U.S. Pat. No. 6,248,157,which is incorporated herein by reference.

While membrane 24 substantially prevents the permeation of liquidousmaterial therethrough, it is understood that solvent vapor may diffusethrough the wall of membrane 24 to permeate side 30 of chamber 16. Asdescribed above, solvent vapor diffusion through membrane 24 may bedriven by differential partial pressures of the solvent vapor as betweenthe retentate and permeate sides of membrane 24. In the case of liquiddegassing systems, mixed-solvent mobile phase may have disproportionatepervaporation rates among each solvent in the mobile phase.Consequently, it is desired to minimize solvent pervaporation acrossmembrane 24, so as to maintain consistent and accurate solvent blends inthe mobile phase.

Henry's Law of Partial Pressure controls the operational parameters incausing gaseous species in the liquid mobile phase to migrate throughgas-permeable membrane 24 to a permeate side 30 of chamber 16. Inparticular, to drive migration across the membrane, permeate side 30exhibits a lower relative concentration or partial pressure of thetarget gaseous species than that found in the liquid mobile phase. Forliquid chromatography applications, the critical gaseous speciesconcentration in the liquid mobile phase is the maximum target gasspecies solute concentration sustainable in the mobile phase withoutoutgassing. For example, methanol and water can each individually holdup to 38% of air without outgassing in any mixture combination of thetwo solvents. As such, the maximum pressure at the permeate side 30 fordegassing air from a methanol/water analyte may be calculated by thefollowing relationship:

P _(degas)=(0.38)(ambient atmospheric pressure)

The ambient atmospheric pressure value must take into account knowndecreases in pressure introduced by the system. For example, flowrestrictions between the mobile phase supply vessels and the mobilephase pump must be deducted from ambient atmospheric pressure in orderto calculate an accurate maximum pressure at permeate side 30 allowablein order to maintain the mobile phase with a gas concentrationsufficiently low to prevent outgassing.

In some applications, however, such a pressure value calculated at alevel only to prevent outgassing of the mobile phase is insufficient toadequately degas the mobile phase. As such, the gas pressure at permeateside 30 required to achieve desired degasification of the mobile phaseis likely to be assessed for each set of operating conditions. Ingeneral, degassing rate is increased with decreased target gas partialpressure on permeate side 30 of chamber 16. To effectuate such anenvironment, permeate side 30 of chamber 16 may be evacuated to arelatively low total absolute pressure by coupling vacuum port 22 to avacuum pump 17.

The equilibrium point pressure at permeate side 30 is calculated as thesum of the vapor pressures of each solvent component in the mobilephase. By operation of Dalton's Law, solvent vapor fills a void space toan extent at which its associated partial pressure meets thecorresponding solvent vapor pressure, when such void space is exposed tothe corresponding solvent. Such an arrangement is present inpervaporation control system 10, wherein only a pervaporation controlspace 12 is available to be filled with solvent vapor up to thecorresponding solvent vapor pressure of a solvent component disposed ata retentate side 32. Pervaporation of the solvents will occur only tothe extent that each solvent vapor fills pervaporation control space 12to a partial pressure equal to its corresponding vapor pressure, atwhich point further pervaporation ceases. Accordingly, Applicants havedetermined that pervaporation of liquid mobile phase from retentate side32 may be limited by minimizing the volume of permeate side 30 ofchamber 16, through the establishment of pervaporation control space 12defined between membrane 24 and a shield member 36, which shield member36 is disposed in permeate side 30 of chamber 16, and interposed betweenmembrane 24 and vacuum port 22. In this manner, the equilibrium pointpressure described above is reached with as little solvent pervaporationas possible. Minimizing the void space defined by pervaporation controlspace 12 provides a variety of other operational advantages, such asrapid pressure stabilization, low volume requirements, and the like.

In one embodiment, shield member 36 defines pervaporation control space12 by establishing a solvent vapor permeability barrier in proximity tomembrane 24. As a result, shield member 36 may exhibit low solvent vaporpermeability, and particularly low permeability to solvent vaporspervaporating from the mobile phase at retentate side 32 of membrane 24.Shield member 36 may therefore exhibit a solvent vapor permeability thatis less than the solvent vapor permeability of membrane 24.

In some embodiments, shield member 36 may be fabricated from one or morepolymeric materials such as FEP, PEEK, Tefzel™, or other suitablematerials. In the illustrated embodiment, shield member 36 is formed asa tube which surrounds tubular membrane 24. Shield member 36 mayconcentrically or nonconcentrically surround membrane 24. Shield member36 may surround tubular membrane 24 to define a substantially closedpervaporation control space 12 between membrane 24 and shield member 36.In some embodiments, shield member 36 may surround tubular membrane 24as tubular membrane 24 extends between inlet connection 26 and outletconnection 28. Shield member 36 may therefore extend continuously frominlet connection 26 to outlet connection 28, such that pervaporationcontrol space 12 is defined continuously from inlet connection 26 tooutlet connection 28.

Shield member 36, however, may be provided in a variety ofconfigurations to establish a desired pervaporation control space 12. Inthe illustrated embodiment, pervaporation control space 12 is definedcontinuously from inlet connection 26 to outlet connection 28. In otherembodiments, however, pervaporation control space 12 may be defined onlyat one or more distinct locations in proximity to membrane 24, such asat locations between inlet connection 26 and outlet connection 28.Shield member 36 may be provided in configurations which are not tubularto suitably define pervaporation control space 12 between membrane 24and shield member 36.

In one particular embodiment, tubular membrane 24 may have an insidediameter “X₁” of 0.011 in., and a wall thickness of 0.005 in. Shieldmember 36 may surround, concentrically or otherwise, tubular membrane 24with an inner diameter “X₂” of 0.030 in, and an outside diameter “X₃” of0.062 in. In such an arrangement, therefore, an average distance betweenmembrane 24 and inner wall 37 of shield member 36 is about 0.007 in. Intypical embodiments, inner wall 37 of shield wall 36 may be spaced frommembrane 24 by less than about 0.03 in. in defining pervaporationcontrol space. In some embodiments, pervaporation control space 12assumes a volume between membrane 24 and shield member 36 that is notgreater than about 30× the volume defined by retentate side 32 ofchamber 16.

It has been discovered by the Applicants that, at a ratio of less thanabout 30:1 (volume of pervaporation control space:volume of retentateside), pervaporation of liquid at retentate side 32 may be limited to anextent which permits relative concentration ranges of a mixed solventsystem within an acceptable error range of chromatographic analysis. Theratio described above, therefore, represents an understanding by theApplicants of empirical evidence of suitably minimized pervaporation. Ithas been further discovered, however, that such ratio may preferably besubstantially less than 30:1, such as less than about 10:1, and evenmore preferably less than about 3:1. To provide a desirably functionalpervaporation control space, both for controlling solvent pervaportiaonacross membrane 24 and for facilitating degassing of the solvent, therelative volume ratio of the pervaporation control space to the spaceddefined on the retentate side of membrane may be at least about 1:1.Relative volumes as between pervaporation control space 34 and retentateside 32 may be established to suit the particular parameters of anoperating system and its associated materials and operating conditions.

It is also to be understood that the relative volume ratios describedabove may not be pertinent for arrangements in which shield member 36and/or tubular membrane 24 are not substantially tubular. Accordingly,it is to be understood that pervaporation control space 12 may bedefined as a limited space between membrane 24 and shield member 36. Intypical embodiments, an average distance between membrane 24 and innerwall 37 of shield member 36 may be at least about 0.001 in, and may bebetween about 0.001 in and about 0.03 in. Such a range has beendetermined by the Applicants to simultaneously facilitate a meaningfullimitation on solvent pervaportaion through membrane 24, and adequatedegassing of the solvent at retentate side 32 of membrane 24.

In order to permit degassing of the liquid mobile phase at retentateside 32 of membrane 24, permeate side 30 of chamber 16 may bedynamically controlled to establish and maintain sufficiently lowpartial pressures of the target species for gaseous removal from theliquid mobile phase. In the context of the vacuum degassing arrangementillustrated in the Figures, therefore, permeate side 30 of the chamber16 may be fluidly coupled to vacuum port 22, such that a vacuum pump mayevacuate permeate side 30 to an extent sufficient to establish andmaintain a target gas partial pressure that effectuates degassing of theliquid mobile phase. Such fluid connection extends to membrane 24, sothat gas removed from the liquid mobile phase through gas-permeablemembrane 24 may be evacuated out from chamber 16 through vacuum port 22.

Conventional degassing systems, such as that described in U.S. Pat. No.6,248,157 establish direct exposure of the permeate side surface of themembrane to a fluid environment that is connected in an unimpededfashion to a vacuum port outlet (see FIG. 2 of U.S. Pat. No. 6,248,157).

The presently described shield member 36 presents a barrier, at least toan extent, for degassed molecules to be removed from chamber 16.Consequently, shield member 36 may be configured to permit limitedbypass of gaseous species while maintaining a substantially closedpervaporation control space 12. A number of approaches may be utilizedto facilitate removal of degassed molecules from chamber 16. Forexample, shield member 36 may be provided with one or more apertures 40which permit limited gas flow therethrough. In the enlarged view of FIG.4, apertures 40 may be slits formed in the wall of shield member 36.Slits 40 may penetrate at least partially through shield member 36, andpreferably establish a pathway for limited gaseous escape through shieldmember 36. Slits 40 may be provided at shield member 36 in any desirednumber, size, or arrangement to provide the desired balance of degas sedvapor outflow from pervaporation control space 12 and the maintenance ofa substantially closed pervaporation control space 12 to limit solventvapor pervaporation pursuant to Dalton's Law. For example, apertures 40may be in the form of slits formed longitudinally substantially parallelto a luminal axis of a tubular shield member 36. It has been found bythe Applicant that such an arrangement for the one or more apertures 40in shield member 36 provides for sufficient degassing efficiency withoutcompromising the structural strength of shield member 36. Applicantscontemplate, however, that the one or more apertures may be provided anyof a number of configurations, including combinations of differentconfigurations. In the example of slits, apertures 40 may be formedlongitudinally, transversely, spirally, or any combination thereof toestablish the desired degree of gas flow out from pervaporation controlspace 12. Accordingly, apertures 40 may be in the form of holes, valves,pathways, and the like. FIGS. 5-7 illustrate example alternativeembodiments for one or more apertures 40 in shield member 36.

In each of the illustrated embodiments, shield member 36 is adapted topermit limited gas flow at least from pervaporation control space 12 toa chamber space 31 of permeate side 30 that is separated frompervaporation control space 12 by shield member 36. In some embodiments,shield member 36 may be adapted to permit gas flow between pervaporationcontrol space 12 and chamber space 31. It is contemplated that shieldmember 35 may be variously configured to achieve the limited gas passagefrom pervaporation control space 12 to chamber space 31. In someembodiments, the one or more apertures 40 in shield member 36 permitsgas passage from pervaporation control space 34 to chamber space 31 onlyupon at least one millimeter Hg absolute pressure differential betweenpervaporation control space 12 and chamber space 31. In typical suchembodiments, therefore, degassing of liquid at retentate side 32 throughgas-permeable membrane 24 that is effectuated by a reduced partialpressure of the target gas at permeate side 30 increases the absolutepressure at pervaporation control space 12 due to the “enclosure effect”of shield member 36 in relation to membrane 24. Shield member 36 may bearranged to permit gas passage from pervaporation control space 12 tochamber space 31 only upon reaching a threshold absolute pressuredifferential, with the absolute pressure at pervaporation control space12 being greater than the absolute pressure of chamber space 31 by thethreshold differential value. As indicated above, such a thresholdabsolute pressure differential may be at least one mm Hg.

In one particular embodiment of the present invention, apertures 40 maycomprise one or more slits substantially longitudinally aligned with acentral luminal axis of a tubular shield member 36, wherein the one ormore slits are of a width, length, and penetration depth to produce anair flow restriction of between about 10-50 SCCM with an absolutepressure at chamber space 31 of about 100 mm Hg. To accomplish such anairflow restriction, the one or more apertures 40 may penetratepartially or completely through a wall of shield member 36. In oneembodiment, for example, a gas passage slit aperture 40 may be producedat shield member 36 by cutting into shield member 36 with a standardrazor blade along an axial direction.

In the illustrated embodiment, a tubular shield member 36/membrane 24assembly may be secured at each of inlet and outlet connections 26, 28with suitable ferrules 27, 29 which are configured to crimpingly engageupon shield member 36 at inlet and outlet connections 26, 28. Respectivenuts 27 a, 29 a operate conventionally to press ferrules 27, 29 intocrimping engagement between body 14 and shield member 36.

To limit air-vapor exchange due to pressure fluctuations within chamber16 caused by the operation of the vacuum pump coupled to vacuum port 22,a pneumatic filtration device may be established by including a flowrestrictor 52 between chamber 16 and the vacuum pump, such as at vacuumport 22. In the illustrated embodiment, flow restrictor 52 is in theform of a capillary tube disposed at vacuum port 22, which capillarytube is suitable for creating a pneumatic pressure oscillation dampenerhaving a time constant that is larger than an oscillation rate of thevacuum pump. The pneumatic pressure oscillation attenuation of flowrestrictor 52 may be calculated by the following relationship:

A=1÷√{square root over (1)}+(2π×F×T)²

Where: F=T_(N)−frequency of fluctuations

-   -   T=(V×128×μ×L)÷(π×d⁴×P)    -   V=chamber volume    -   μ=dynamic viscosity of air    -   L=restrictor length    -   d=restrictor inside diameter    -   P=pressure

Flow restrictor 52 is preferably configured to be effective in reducingpressure fluctuations in chamber 16 caused by the reciprocaldisplacement operation of the vacuum pump. The time constant of flowrestrictor 52 and chamber 16 should therefore be larger than theoscillation rate of the positive displacement effected through thereciprocating piston of vacuum pump 17. In a particular embodiment, flowrestrictor 52 is configured to permit up to about 1 mm Hg absolutepressure differential thereacross. In one embodiment, flow restrictor 52is a capillary tube having an inside diameter of 0.01 in, and length of0.5 in, wherein the volume of chamber 16 is about 28 cm³. Flowrestrictor 52 and chamber 16, however, may be provided in a variety ofconfigurations to meet the pneumatic pressure oscillation dampeningperformance of the present invention.

While flow restrictor 52 may be effective in reducing pressurefluctuations in chamber 16, such pressure fluctuations may neverthelessbe imparted upon pressure sensor 70 between flow restrictor 52 andvacuum pump 17. Such pressure oscillations detected by sensor 70 may,without corrective programming to controller 72, cause controller 72 todrive vacuum pump 17 at varying speeds in an effort to maintain apressure set point in chamber 16. The changing speeds of vacuum pump 17may exacerbate pressure oscillations, both detected by pressure sensor70, and within degassing chamber 16. In fact, in a closed-loop controlscheme for maintaining a pressure setpoint within degassing chamber 16,the pressure oscillations detected by pressure sensor 70 may drive thesystem to actually create pressure oscillations within degassing chamber16 which overcome the dampening properties of flow restrictor 52,thereby potentially leading to undesired pervaporation.

To reduce pressure oscillations, both within degassing chamber 16 and atsensing point 68 of pressure sensor 70, a pneumatic filtration apparatus74 may be operably positioned between vacuum pump 17 and sensinglocation 68. The pneumatic filtration apparatus 74 includes one or moreof a volume exchange chamber 76 and a flow restrictor 78. In theembodiment illustrated in FIG. 8, volume exchange chamber 76 is disposeddownstream from flow restrictor 78. However, it is contemplated that therelative positions of exchange chamber 76 and flow restrictor 78 may bereversed, wherein flow restrictor 78 is downstream from exchange chamber76. For the purposes hereof, the terms “downstream” and “upstream” areintended to refer to the gas flow direction in degassing line 71 fromdegassing chamber 16 to exhaust 80 of vacuum pump 17. Thus, a componentdisposed “downstream” from another component or location in thepervaporation control system is proximally disposed to exhaust 80 withrespect to the other component or location.

Volume exchange chamber 76 and flow restrictor 78 may work individuallyor in combination to attenuate pressure oscillations upstream frompneumatic filtration apparatus 74. Consequently, the attenuationcontribution of each of exchange chamber 76 and flow restrictor 78 maybe assigned in the construction of pneumatic filtration apparatus 74 tooptimally perform in the respective pervaporation control/degassingsystem. In one aspect, for example, flow restrictor 78 may be limited inits attenuation contribution by the pressure drop thereacross.Specifically, the pressure drop magnitude across flow restrictor 78 ispreferably of a magnitude within desired operating ranges for vacuumpump 17, such that vacuum pump 17 possesses the capability to maintainthe pressure setpoint within degassing chamber 16 even through thechange in pressure caused by flow restrictor 78. In some cases, flowrestrictor 78 is designed to contribute to pressure oscillationattenuation while not significantly increasing the power draw (speed) ofvacuum pump 17. In one embodiment, flow restrictor 78 may be a capillarytube with an inside diameter that is substantially smaller than anominal inside diameter of degassing line 71 coupling degassing chamber16 to vacuum pump 17. For example, the capillary tube of flow restrictor78 may have an inside diameter of 0.01 in. and a length of 2 in.However, it is to be understood that capillary tubes of variousdimensions, as well as a variety of other configurations or devices maybe utilized to achieve the desired flow restriction.

Volume exchange chamber 76 of pneumatic filtration apparatus 74 may beprovided as an added “dead space” volume between sensor location 68 andvacuum pump 17. Such volume of exchange chamber 76, as well as the flowrestriction of flow restrictor 78, operate individually or incombination to provide pneumatic pressure oscillation attenuation inaccordance with the relationship described above. Namely, theattenuation provided by pneumatic filtration device 74 may be calculatedwith “V′ being the volume of exchange chamber 76, and “L” and “d” beingthe diameter and length dimensions, respectively, of flow restrictor 78.The pressure oscillation attenuation provided by pneumatic filtrationapparatus 74 may be at least 10%, and, in some embodiments, at least50%.

It is contemplated that exchange chamber 76 may be provided in any of avariety of configurations, including a separate chamber body fluidlycoupled to degassing line 71, or a widened and/or lengthened degassingline 71. As expressed in the above relationship, the increased volumebeing pumped by vacuum pump 17, and in some embodiments downstream fromsensor location 68, contributes to the attenuation of pressureoscillations within degassing line 71 and degassing chamber 16. In oneembodiment, exchange chamber 76 may be in the form of an extended lengthof degassing line 71 for a total added volume of 5-10 milliliters. Othervolumes and configurations for exchange chamber 76, however, arecontemplated by the present invention.

Some liquid degassing systems involve a plurality of distinct degassingchambers assigned to degas distinct liquid mobile phase streams. In somecases, such distinct mobile phase streams may contain identical mobilephase compositions. In other cases, however, such mobile phase streamsmay carry distinct mobile phase compositions. Typically, vacuumdegassing of such plurality of degassing chambers is effected through asingle vacuum pump fluidly coupled to each of the degassing chambersthrough a manifold. It has been discovered that pressure oscillationscaused by the reciprocating characteristic of the vacuum pump can inducepervaporated solvent from one chamber to be retrogradedly transferredinto a different vacuum chamber through the manifold. In particular,increased pressure in the manifold during the intake stroke of thevacuum pump can cause air and pervaporated solvent to “reverse” coursefrom the manifold into a degassing chamber.

To limit or prevent such “cross-contamination” of pervaporated solventinto the degassing chambers, volume buffer chambers may be disposedbetween the manifold and the respective degassing chambers. An exampleembodiment of the present invention is illustrated in FIG. 9, whereinpervaporation control system 110 includes a plurality of liquiddegassing apparatus 108 a-108 d, each including a degassing chamber 116a-116 d, and a flow restrictor 152 a-152 d in similarity to thatdescribed above with reference to pervaporation control system 8.However, it is to be understood that degassing apparatus 108 a-108 dneed not be identical, or even similar to one another, nor withdegassing apparatus 8. Pervaporation control system 110 further includesa manifold 186 fluidly coupling individual degassing lines 171 a-171 dwith main degassing line 171. Manifold 186 may comprise any suitableconfiguration for coupling respective degassing lines 171 a-171 d into asingle degassing line 171 for coupling to vacuum pump 117. In oneembodiment, manifold 186 may include a combination of tubing andconnection joints for establishing the respective degasser connections104 a-104 d to manifold 186, and a connection fluidly coupling manifold186 to degassing line 171. In some embodiments, pervaporation controlsystem 110 includes a pressure sensor 170 for detecting pressure atsensor location 168, a controller 172 for controlling vacuum pump 117based upon signals generated by pressure sensor 170, and a pneumaticfiltration device 174 including a volume exchange chamber 176 and a flowrestrictor 178.

As indicated above, pervaporation control system 110 may include volumebuffer chambers 190 a-190 d fluidly coupled between respective degassingchambers 116 a-116 d and vacuum pump 117. In the illustrated embodiment,volume buffer chambers 190 a-190 d may be fluidly coupled to therespective individual degassing lines 171 a-171 d between the respectiveflow restrictors 152 a-152 d and manifold connections 104 a-104 d. It isto be understood, however, that pervaporation control system 110 mayincorporate one or more volume buffer chambers in any of a variety ofarrangements and configurations to suitably inhibit cross-contaminationof pervaporated solvent into one or more of the degassing chambers 116a-116 d. Therefore, the illustrated embodiment of volume buffer chambers190 a, 190 d is merely exemplary, and may specifically alternativelyinclude one or more buffer chambers disposed respectively upstream fromflow restrictors 152-152 d, or downstream from manifold connections 104a-104 d.

In order to effectively inhibit the pervaporated solventcross-contamination described above, volume buffer chambers 190 a-190 dmay be appropriately sized and configured to diminish pressureoscillations at respective degassing lines 171 a-171 d that are causedby the intake/exhaust stroke cycles of vacuum pump 117. In a particularembodiment, the volume of buffer chambers 190 a-190 d may be selected toexceed a theoretical intake volume to the respective degassing chamber116 a-116 d as a result of the pressure oscillation. The followingrelationship is used to calculate the theoretical intake volume to eachdegassing chamber 116 a-116 d:

V _(i)=[(P _(o) /P _(set))×(100%)]×V _(ch),

Wherein,

-   -   V_(i)=intake volume    -   P_(o)=pump pressure oscillation    -   P_(set)=pressure set point in chamber    -   V_(ch)=degassing chamber volume

In one embodiment, therefore, each of buffer chambers 190 a-190 d may beconfigured with a volume which exceeds the respective intake volume(V_(i)). However, it is to be understood that the volumes for bufferchambers 190 a-190 d may be equal or inequal, and one or more of suchbuffer chambers 190 a, 190 d may not exceed an intake volume of anassociated degassing chamber 116 a-116 d. In a particular embodiment,buffer chambers 190 a-190 d may be provided as an added “dead space”volume in the respective degassing lines 171 a-171 d, and may beprovided in any of a variety of configurations, including a separatechamber body fluidly coupled to a respective degassing line 171 a-171 d,or a widened and/or lengthened degassing line 171 a-171 d. In oneembodiment, the buffer chamber volume may be in the form of an extendedlength of a respective degassing line for a total added volume of 0.2-1ml.

The invention has been described herein in considerable detail in orderto comply with the patent statutes, and to provide those skilled in theart with the information needed to apply the novel principles and toconstruct and use embodiments of the invention as required. However, itis to be understood that the invention can be carried out byspecifically different devices and that various modifications can beaccomplished without departing from the scope of the invention itself.

1. A liquid degassing apparatus comprising: a degassing module fordegassing a liquid conveyed through a chamber thereof, said degassingmodule including a gas-permeable, liquid-impermeable membrane separatingsaid chamber into a permeate side and a retentate side, wherein saidretentate side of said chamber is in liquid communication with a liquidinlet and a liquid outlet for the liquid, and said permeate side of saidchamber is in fluid communication with a vacuum port of said degassingmodule; a vacuum pump fluidly coupled to said vacuum port of saiddegassing module through a degassing line to evacuate said permeate sideof said chamber; a pressure sensor for sensing pressure of said permeateside of said chamber; a controller communicatively coupled to saidpressure sensor and said vacuum pump for controlling vacuum pumpoperation responsive to signals received from said pressure sensorindicating pressure of said chamber; and a pneumatic filtrationapparatus fluidly interposed between said vacuum pump and said pressuresensor within said degassing line, said pneumatic filtration apparatusincluding one or more of a first flow restrictor and a volume exchangechamber defining a volume open to said degassing line, said pneumaticfiltration apparatus being capable of attenuating pressure oscillationsdeveloped in said degassing line by said vacuum pump, wherein theattenuation provided by said pneumatic filtration apparatus is at least10%.
 2. A liquid degassing apparatus as in claim 1 wherein theattenuation provided by said pneumatic filtration apparatus is at least50%.
 3. A liquid degassing apparatus as in claim 1 wherein saidpneumatic filtration apparatus includes said volume exchange chamberdownstream from said first flow restrictor.
 4. A liquid degassingapparatus as in claim 1, including a second flow restrictor disposedupstream from said pressure sensor within said degassing line.
 5. Aliquid degassing apparatus as in claim 4 wherein said second flowrestrictor is suitable for creating a pneumatic pressure oscillationdampener having a time constant that is larger than an oscillation rateof said vacuum pump.
 6. A liquid degassing apparatus, comprising: aplurality of degassing modules for degassing one or more liquidcompositions, each of said degassing modules including a chamberseparated by a gas-permeable, liquid-impermeable membrane into apermeate side and a retentate side, wherein said retentate sides of saidchambers are liquidly disconnected from one another; a manifold fluidlyconnecting said permeate sides of said chambers through respectiveindividual degassing lines individually extending between said manifoldand respective vacuum ports in fluid communication with said permeatesides of said chambers, said manifold further fluidly connecting saidindividual degassing lines with a main degassing line at a connection; avacuum pump fluidly coupled to said main degassing line for evacuatingsaid permeate sides of said chambers; and a buffer chamber fluidlyinterposed between said connection and at least one of said degassingmodules within a respective individual degassing line, said bufferchamber defining a first volume open to said individual degassing line,which first volume is of a magnitude exceeding an intake volume, whereinthe intake volume is defined as:[(P_(o)/P_(set))×(100%)]×V_(ch) wherein, P_(o)=pump pressure oscillationof said vacuum pump; P_(set)=a pressure set point in said permeate sideof said respective degassing module; and V_(ch)=a volume of saidrespective degassing module chamber.
 7. A liquid degassing apparatus asin claim 6, including a buffer chamber fluidly interposed between saidconnection and each of said degassing modules within respective saidindividual degassing lines.
 8. A liquid degassing apparatus as in claim6, including a flow restrictor fluidly interposed between said bufferchamber and said degassing module in said individual degassing line.